Portable Document Format (PDF) is a file format developed by ADOBE SYSTEMS INCORPORATED that allows documents formatted in PDF to be retrieved for on-line viewing, using a computer monitor for example, in the same manner that hard copies of the documents would be viewed off-line. For example, PDF documents can be viewed page by page and scrolled through on-line according to particular needs of users. To view PDF documents, applications such as ADOBE ACROBAT EXCHANGE or ADOBE ACROBAT READER are typically required. Using such applications in content-based document retrieval operations, where large numbers of PDF documents may need to be retrieved for viewing on an ongoing basis, significant time and effort may be expended in determining whether retrieved PDF documents are duplicates (same document previously retrieved), updates (changes to previously retrieved document), or new (first time document retrieved).